The frictional resistance of a vessel hull as it moves through the water can constitute 45% to 90% of the total resistance and may be increased by 6% up to 80% due to the fouling of the hull by algae, sea grass, barnacles, and the like. An added resistance of 30% due to moderate bio-fouling of a tanker hull can increase the fuel consumption of the vessel by twelve tons per day. The result is added cost to operate the vessel and increased emissions.
Accordingly, there are a variety of methods employed to lower the chance of bio-fouling and/or to clean the hull of vessels. For example, hull paints and coatings are used in an effort to decrease the chance of bio-fouling, but such treatments do not always work reliably. See, for example, U.S. Pat. No. 7,390,560 incorporated by reference herein. Also, the vessel must be dry docked for an extensive period of time while the paint and/or coating is applied. There are also environmental concerns with anti-fouling paints and coatings.
So, typically, while the ship is dockside and/or during normal unlading conditions, the hull is periodically cleaned manually by scuba divers using motorized brushes. The cost of such a cleaning procedure is high This type of cleaning effort is repeated approximately every ten to twenty months or sooner if needed. Worse, some jurisdictions have made this practice illegal due to the toxicity of anti-fouling paint which contaminates the water.
In response, robotic hull cleaners have been proposed. The “Hismar” consortium, for example, has proposed a robotic platform for hull cleaning during normal unlading conditions. The robot is magnetically attached to the hull when the vessel is stationary and is tethered to an operator control unit, a high pressure water source, a suction subsystem, and a power subsystem.
Other tethered robots are disclosed in U.S. Pat. Nos. 5,628,271; 6,317,387; 4,674,949; 4,401,048; 4,079,694; 3,946,692; and 3,638,600. See also WO 02/074611; “Design of a Remotely Operated Vehicle (ROV) for Underwater Ship Hull Cleaning,” by Lee Min Wai Serene and Koh Cheok Wei; and “The Design of Underwater Hull-Cleaning Robot,” by Fu-cai et al., published in The Journal of Marine Science and Application, Vol. 3, No. 1, June 2004. All of these references are hereby incorporated by reference herein.
Most prior hull cleaning robots suffer from several shortcomings. Typically, the robots are connected to a cable and powered and controlled by an on-board power supply and control subsystem and are able to operate only on a stationary vessel.